Such a circuit, which is known as a circuit with a bridge configuration, provides great flexibility and avoids operating the circuit in conditions of high distortion.
One simple bridge circuit thus splits the signal into two components having a phase difference therebetween of 90.degree. or 180.degree., using a power divider. One of the signals is then treated by a linear branch forming the first stage and the other signal by a non-linear circuit forming the second stage. The phase and amplitude of the signal are not altered in the linear branch. However, the phase and amplitude are modified in the non-linear branch. The two signals are then combined with suitable amplitude and phase, obtained with an attenuator and a phase shifter for example, in a combining circuit which is in general of the same type as the circuit used to split the input signal. The result of this treatment is that the signal presented to the input of the linearization circuit can have gain expansion and a delay, or even an advance, which can be made complementary to those of the transmitter which is to be linearized.
Such a pre-distorting linearization circuit can be used in particular for satellite transmissions involving a plurality of carriers. It is an object of the invention to allow the power amplifiers used for this transmission to operate closer to their saturation characteristics, which improves their efficiency and thus their functionality. Such a consideration is equally applicable to transmitters for amplifying signals with a non-constant envelope, for example with filtered QPSK modulation, i.e. quadrature modulation with phase shift.
Pre-distorting linearization circuits are used in preference to linearization circuits of other types because of their efficiency, their simplicity, and their low losses, and also because they do not introduce attenuation into the output signal.
A pre-distorting linearization circuit can also be useful in distributed antenna systems (array, multi-port systems, etc.) in order to increase linearity and at the same time reduce the phase and gain tracking demands between the different amplifier units.
A pre-distorting linearization circuit is generally located between the amplifier of the corresponding channel and the traveling wave tube amplifier, or is even integrated directly into an integrated power amplifier, to form an amplifier module. Its function is to provide amplitude and phase characteristics which complement those of the transmitter, so as to compensate for distortion.
The end result which can be achieved with this linearization is that the transmitter can be operated very close to its saturation region while preserving satisfactory nominal linearity.
Known types of linearization circuits which are in the form of a bridge generally have a linear branch formed by a simple connection, by a phase shifter and/or by a variable attenuator or even an amplifier. The non-linear branch is implemented by a component which introduces distortion, such as a PIN or Schottky diode or even MESFET type field effect transistors with a single or double gate.
One of the important parameters of linearization circuits is the amplitude ratio of the bridge, i.e. the ratio between the two output signals of the two branches. Ideally this ratio should be as close as possible to 0 dB.